Lighting industry is rapidly moving to LED-lighting systems. Efficiency and power output of a typical LED has risen quickly. Epoxides have traditionally been used as an encapsulant but the conventional materials can no longer handle the intense light flux and heat that state-of-the-art LEDs generate. Dimethylsilicone (PDMS), a known encapsulating material for electronic devices, has recently been used more and more as an encapsulant for LEDs due to better durability and resistance to yellowing than epoxides.
As far as the use of dimethylsilicone is concerned reference is made to U.S. Pat. Nos. 4,278,784, 6,492,204 and 6,806,509.
However, PDMS has a relatively low refractive index (RI ˜1.4) compared to LED-chip (‘epi’, e.g. InGaN, RI˜2.5) and many phosphor materials used in white LEDs (for example yttrium aluminium garnet, ‘YAG’, RI˜1.85). This refractive index mismatch creates internal reflections, which lower the light output and efficiency of the device. Replacing PDMS with higher RI phenyl silicones (RI˜1.50 . . . 1.55) somewhat improves the situation but there is still a need for even higher RI materials that can withstand the conditions inside LEDs without yellowing and are suitable for LED manufacturing.